1. Field of the Invention
This invention relates to seals for preventing leakage of a fluid about a shaft extending through the housing of a rig, such as a pump. The invention is particularly concerned with a seal for containing high vapor-pressure liquids, such as liquefied light hydrocarbon gases and the method of sealing such fluids.
2. Background Art
Two types of spaced apart mechanical end face seal systems have been used in the past for containing high vaporpressure liquids. These systems were of the "wet," contacting type which had to be cooled by a buffer liquid pumped between two axially spaced mechanical seal faces. The system for circulating the buffer liquid (frequently oil) includes pumps, coolers and reservoirs which add complexity and expense to the apparatus. Further, the mechanical seals, even with their complicated support system, often last only a few days in pumps handling liquefied light hydrocarbons.
It is known that another type of mechanical end face seal, known as a dry running gas seal, can be operated without oil lubrication when one of the faces is provided with a series of spiral grooves. Seals of this type are shown and described in U.S Pat. No. 4,212,475, issued to Josef Sedy and assigned to the present assignee. The subject matter of this patent is incorporated by reference herein. This latter type of seal has been used to solve gas containment problems. In U.S. Pat. No. 4,212,475, Sedy discloses and claims optimum parameters for dry running gas seals. These parameters include a dam width ratio, groove depth, groove angle and balance ratio. The seal described in Sedy is directed to the containment of fluids which are already in a gaseous state and under high pressure.
This invention seeks to provide an end face seal arrangement which is particularly appropriate for use in equipment for handling liquids having high vapor-pressure, such as liquefied light hydrocarbons. This is accomplished in part by ignoring, to some extent, the conventional wisdom which is to cool the opposing faces of the seal. Instead, the seal is intended to create shear heating of the sealed fluid between the seal faces of a first stage or module which, it is believed, causes phase change of the liquid to a gas, thus providing a sealing capacity. An optional second stage dry running gas seal may also be used to effectively contain the vaporized fluid.
Generally, the ratio of circumferential length of the grooved surfaces relative to that of the ungrooved surfaces, disposed between the groove surfaces and known as lands, is an important parameter of seal face design. This land/groove ratio is commonly a one-to-one ratio.
In the radial direction, an annular band comprising grooves and lands form a groove portion at one radial face edge and a smooth, totally ungrooved annular band at the other radial face edge of the sealing face. The ratio of radial width of the dam portion to the groove portion is referred to as the dam width ratio. A dam width ratio of one-to-one is commonly used in mechanical seals. Sedy discloses and defines a dam width ratio of between approximately 0.5 and 0.8 in U.S. Pat. No. 4,212,475.
These ratios of radial and circumferential dimensions are sufficient to provide a noncontacting mechanical face seal for sealing of gases. However, special considerations are necessary when sealing fluids such as highly volatile liquids. For example, liquid natural gas is in gaseous form at normal temperatures and pressures. Under higher pressures and lower temperatures, natural gas is in liquid form. The mechanical face seals and sealing face designs presently in use may be improved according to the present invention by providing for increased shear heating of the liquid natural gas when it is passing through the seal. Shear heating raises the temperature of the liquid natural gas and converts it into a gas, which is easier to seal and which will minimize leakage.
One approach to providing a face seal configuration for a high vapor-pressure liquid application is proposed in U.S. Pat. No. 4,889,348, issued to Amundson et al and assigned to the present assignee. The subject matter of that patent is incorporated by reference herein where appropriate. A dual seal module system using an inboard, upstream and an outboard downstream module, one each mounted on the shaft and housing, has been proposed by Amundson et al. Each seal module has a primary ring affixed to the housing and a mating ring affixed to the shaft. The rings have opposing, radially extending faces, one of which has a plurality of downstream pumping spiral grooves extending from one circumference of the seal face. The grooves of at least the ring of the upstream module have a depth which was described as being no more than about 100 micro-inches. The depth of grooves of the second or downstream module were recited as being not less than the depth of grooves of the upstream module. The downstream module groove depth was also described as being no more than about 200 micro-inches.